This invention relates to a catalyst and process for increasing the yield of iso-olefinic hydrocarbons in a fluid catalytic cracking unit (FCC). More particularly, this invention relates to a modified FCC catalyst and a process for using such a catalyst wherein the catalyst is modified by trivalent cation exchange with an ion exchange solution comprising a trivalent cation, a trivalent cation complexing agent, and a hydroxide-producing component. Increased yields of FCC unit olefinic hydrocarbons are particularly useful for the production of oxygenated gasoline blending components.
Oxygenates have been part of the United States gasoline strategy since the late 1970s. With the Clean Air Act Amendments of 1990, the demand for oxygenates is expected to increase even further. For example, starting in the winter months of 1992, gasoline containing 2.7 weight percent oxygen will have to be provided to approximately 40 metropolitan areas that have failed to meet carbon monoxide pollution standards. It is expected that in the near future, between 30 and 60 percent of the United States gasoline pool may be required to contain oxygenates. Current oxygenate production capacity is insufficient for meeting these requirements.
The most commonly used oxygenates today are methanol, ethanol, and methyl tertiary butyl ether (MTBE). Although methanol and ethanol have high blending octanes, problems with toxicity, water miscibility, high Reid Vapor Pressure (RVP), high nitrogen oxide emissions, lower fuel efficiency, and cost have dampened industry enthusiasm for these components. Partially as a result of the above, MTBE has become particularly attractive.
Homologues of MTBE such as ethyl tertiary butyl ether (ETBE) and methyl tertiary amyl ether (TAME) are also gaining industry acceptance. Moreover, commercial activity with respect to ETBE and TAME is expected to increase relative to MTBE, in view of the recent Environmental Protection Agency decision to reduce the RVP requirements for gasolines well below 9 psia, the blending RVP of MTBE.
Oxygenate production in the United States is generally limited by oxygenate plant capacity and by feedstock availability. MTBE and ETBE both utilize isobutylene as a feedstock while TAME utilizes isoamylene as a feedstock. Isobutylene and isoamylene are generally supplied to petroleum refinery MTBE, ETBE, and TAME facilities from fluid catalytic cracking units, fluidized or delayed cokers, and/or from downstream paraffin dehydrogenation and isomerization facilities. The availability of hydrocarbons having 4 or 5 carbon atoms is generally limited by constraints such as, but not limited to, crude properties, commercial FCC catalyst properties, FCC operating conditions, and coking conditions, etc. The chemical mix of C.sub.4 and C.sub.5 paraffins, olefins, and aromatics as well as the particular mix of iso-olefins to normal olefins are similarly constrained.
It has now been found that another method exists to favorably affect the yield of olefins and iso-olefins for use in such oxygenate producing processes. FCC catalysts comprising crystalline molecular sieves can be modified either during or after production of the catalyst off-site, or on-site at a fluid catalytic cracking facility, in order to favorably increase the yields of olefins and, in particular, iso-olefins having 4 and 5 carbon atoms.
Crystalline molecular sieves commonly utilized in fluid catalytic cracking catalysts have distinct crystal structures which are demonstrated by X-ray diffraction. The crystal structure defines cavities and pores which are characteristic of the different species. The adsorptive and catalytic properties of each molecular sieve are determined in part by the dimensions of its pores and cavities. Thus the utility of a particular molecular sieve as a fluid catalystic cracking catalyst depends at least partly on its crystal structure. Although many different molecular sieves have been disclosed in the prior art for use in cracking catalysts, there continues to be a need for still more effective molecular sieves and methods for making and modifying such sieves. This is particularly true since the product needs of the refiner continue to change due to unpredictable and quickly evolving events such as, for example, unforeseen nor predictable environmental regulation.
Synthetic molecular sieves are often prepared from mixtures containing alkali metal hydroxides and therefore, can have alkali metal contents of 1 percent by weight or more. The ion exchange of various metals or ammonium ion for such alkali metals is generally performed in catalysis in order to obtain active sites that will facilitate particular catalytic reactions. For example, ion exchange of other metal cations or ammonium ion for such alkali metals can be performed to modify catalyst acidity subject to the particular reactions desired and the feedstock and operating condition constraints inherent to the process for conducting these reactions. In other cases, ion exchange of other metal cations or ammonium ion for such alkali metals can be performed to obtain a particular type of activity or selectivity conducive to catalyzing a particular type or degree of reaction. Typical exchangeable alkali metals in the sieve include sodium or potassium and such an ion exchange can be performed with components such as ammonium nitrate or acetate, followed by a subsequent heating step for releasing ammonia, wherein a proton remains at the exchangeable site. This type of ion exchange generally leaves the molecular sieve in the "hydrogen form."
For purposes of the present invention, the term "ion exchange" shall mean the method of changing one cation for another cation at the ion exchangeable sites in the pores of the molecular sieve. This term does not refer to the elemental replacement of one framework element by another potential framework element. Framework elements are generally those elements that are tetrahedrally bonded through oxygen to each other for providing the typical molecular sieve framework.
Similarly, the term "ion exchangeable sites" shall means the site(s) in a molecular sieve occupied by the cation that balances the negative charge of the electron rich framework tetrahedra.
Metal cation ion exchanges such as aluminum ion exchange can also be performed and generally involve the addition of a molecular sieve to an ion exchange solution comprising a metal salt such as aluminum nitrate and water as exemplified by the following example:
Below a pH of about 4 EQU Al(NO.sub.3).sub.3.9H.sub.2 O (s).fwdarw.Al(H.sub.2 O).sub.6.sup.+3 +3NO.sub.3.sup.-1 (sol)+3H.sub.2 O EQU Al(H.sub.2 O).sub.6.sup.+3 .fwdarw.Al(OH)(H.sub.2 O).sub.5.sup.+2 +H.sup.+ EQU NaMS(s)+Al.sup.+3 (sol).fwdarw.Al MS (s)+3Na.sup.+1 (sol)
where MS is a molecular sieve, Na and Al are the sodium and aluminum ions respectively, and where (s) and (sol) designate the solid species and species dissolved in solution respectively.
Trivalent cation ion exchange can be particularly beneficial, compared to divalent and monovalent cation ion exchange, due to advantages in molecular sieve stability and enhanced activity and selectively.
However, it is generally known in the prior art that trivalent cation ion exchanges with aluminum can be very difficult to effect. See Carvajal, Chu, and Lunsford, The Role of Polyvalent Cations in Developing Strong Acidity: A Study of Lanthanum-Exchanged Zeolites, Journal of Catalysis 125, 123-131 (1990). In Dealumination of Large Crystals of Zeolite ZSM-5 by Various Methods by Kornatowski, Rozwadowski, Schmitz, and Cichowlas, J. Chem. Soc., Faraday Trans., 88(9), 1339-43, it is noted that ion exchange of ZSM-5 with Al.sup.+3 by using aqueous solutions of Al salts is impossible. The salts of metallic cations, and particularly the trivalent cations such as aluminum, generally form acidic solutions when dissolved in water. For example, the pH of aluminum nitrate generally ranges from about 1 to about 3. Maintaining a low trivalent cation ion exchange solution pH is generally necessary to keep the aluminum in solution. Where the pH of the ion exchange solution comprising an aluminum trivalent cation is increased beyond a level of about 4, the lower solution acidity creates a competition between the aluminum ion exchange reaction and hydroxide ion wherein the aluminum cation can form the colloidal hydroxide and precipitate from the ion exchange solution according to the following reactions:
Above a pH of about 4 EQU Al(NO.sub.3).sub.3.9H.sub.2 O(s).fwdarw.Al(H.sub.2 O).sub.6.sup.+3 +3NO.sub.3.sup.-1 (sol)+3H.sub.2 O EQU Al(H.sub.2 O).sub.6.sup.+3 .fwdarw.Al(OH).sub.3 (s)+3H.sub.2 O+3H.sup.+
It is generally the formation of the aluminum hydroxide precipitate that will not allow aluminum ion exchange to occur above a pH of about 4. Therefore, practicality has historically dictated that such molecular sieve ion exchanges using trivalent cations be conducted at a pH of below 4.
However, molecular sieve ion exchange using ion exchange solutions having a pH of less than 4 can cause the framework aluminum of a zeolite to be acidically leached from the silicon framework. For non-zeolitic molecular sieves such as a borosilicate or a gallosilicate, the framework boron or gallium can similarly be acidically leached from the silicon framework. Since this leaching effect generally reduces the number of exchangeable sites in the molecular sieve, the level of possible trivalent cation ion exchange is also reduced. This leaching effect generally results in a less acidic sieve and can be undesirable to the fluid catalytic cracking catalyst manufacturer or fluid catalytic cracking process operator. Moreover, with some zeolites, a pH of less than 4 can cause the general collapse of the framework and result in an amorphous material.
Therefore, there is a great need in fluid catalytic cracking and catalysis in general, for a method for trivalent cation ion exchange of fluid catalytic cracking catalysts comprising at least one molecular sieve that avoids the problems inherent to the methods of the prior art and does not acidically leach framework metals from the molecular sieve.
Conventional methods for the ion exchange of molecular sieves, and in particular, the zeolites are disclosed in Zeolite Molecular Sieves, Donald W. Breck, John Wiley & Sons at pages 529-580 (1974).
Ion-exchange of cations into zeolite, and particularly the Y zeolite, has also been studied extensively, including work by H. S. Sherry in J. Phys. Chem. (1968) 72, 4086 and in Advan. Chem. Ser. (1971) 101, 350.
Lactic acid has been used in catalysis for templating zeolite synthesis. For example, U.S. Pat. No. 4,511,547 to lwayama et al. and U.S. Pat. No. 4,581,216 to lwayama et al. disclose the use of lactic acid for the formation of zeolites where the cation-lactate is a space-filling material around which the zeolite is crystallized. The lactic acid in the lwayama et al. references is provided for templating the zeolite during formation and is not used for zeolite modification or for the ion exchange of the zeolite subsequent to formation.
Enhancing the activity of a zeolite through low temperature steaming has been disclosed for limited uses. For Example, U.S. Pat. No. 4,784,747 to Shihabi discloses a catalytic hydrodewaxing process utilizing a catalyst comprising an aluminosilicate crystalline zeolite having a silica to alumina ratio of at least 250:1 and a Constraint Index from 1 to 12 and an alumina binder. The catalyst is subsequently steamed by heating for at least one hour in the presence of water at a temperature from about 300.degree. C. to 500.degree. C. such that the pressure during steaming ranges from about 100 to 500 kPa.
It has now been found that trivalent cation ion exchange of fluid catalytic cracking catalysts comprising one or more molecular sieves can be performed while minimizing the adverse effects of framework metal leaching by complexing the trivalent cation in a manner so as to keep it from precipitating from the ion exchange solution when the pH of the solution is increased.
It has also been found that increasing the pH of the ion exchange solution to a level ranging from about 4 to about 8, as provided for in the present invention, results in effective ion exchange while substantially minimizing leaching of the framework metal and the reduction in ion exchangeable sites caused by such leaching.
It has also been found that when using the particular complexing solution of the present invention comprising one or more of the alpha, beta, and gamma hydroxy- and amino-carboxylic acids and some crown ethers as exemplified by lactic acid, tartaric acid, glycine, and 15-crown-5, and equivalents thereof, the complexed trivalent cation can generally continue to enter the pores of the molecular sieve and gain access to the exchangeable sites.
It has also been found that when using the particular complexing solution of the present invention comprising one or more of the alpha, beta, and gamma hydroxy- and amino-carboxylic acids and some crown ethers as exemplified by lactic acid, tartaric acid, glycine, and 15-crown-5, and equivalents thereof, the complexed trivalent cation can be effectively released to the molecular sieve exchangeable sites of the fluid catalytic cracking catalyst.
It has also been found that when a fluid catalytic cracking catalyst is modified utilizing the method of the present invention, the yield of iso-olefins, and particularly iso-olefins having 4 and 5 carbon atoms, can be increased compared to identical fluid catalytic cracking catalysts that are not so modified.
It is therefore an object of the present invention to provide a catalyst and process for fluid catalytic cracking comprising the effective trivalent cation ion exchange of fluid catalytic cracking catalysts comprising molecular sieves.
It is another object of the present invention to provide a catalyst and process for fluid catalytic cracking comprising the effective trivalent cation ion exchange of fluid catalytic cracking catalysts comprising molecular sieves at a pH above 4.
It is another object of the present invention to provide a catalyst and process for fluid catalytic cracking comprising the trivalent cation ion exchange of fluid catalytic cracking catalysts comprising molecular sieves which reduces the level of acidic leaching of framework metals and permits the molecular sieve to retain its integrity and maintain its cracking activity compared to prior art processes.
It is yet another object of the present invention to provide a catalyst and process for fluid catalytic cracking comprising the trivalent cation ion exchange of fluid catalytic cracking catalysts comprising molecular sieves which increases the yield of iso-olefins produced in a fluid catalytic cracking process compared to prior art catalysts and processes.
Other objects appear herein.